The invention proceeds from a fluorescent material from the class of the silicide nitrides in accordance with the preamble of claim 1. In particular these are silicide nitrides which fluoresce in the yellow region.
Fluorescent materials of the silicide nitride type such as Sr2Si5N8 and Ba2Si5N8, already known from the article by Schlieper, Millus and Schlick: Nitridosilicate II, Hochtemperatursynthesen and Kristallstrukturen von Sr2Si5N8 und Ba2Si5N8 [Silicide nitrides II, high-temperature syntheses and crystal structures of Sr2Si5N8 and Ba2Si5N8], Z. anorg. allg. Chem. 621, (1995), page 1380. However, in this case no activators are specified which would suggest efficient emission in specific regions of the visible spectrum.
It is an object of the present invention to provide a flourescent material from the class of the suicide nitrides having a cation and the basic formula AxSiyNz, the efficiency of which is as high as possible, and which can be effectively stimulated by UV radiation in the region of 370 to 430 nm.
This and other objects are attained in accordance with one aspect of the invention directed to a highly efficient fluorescent material from the class of the silicide nitrides having a cation and the basic formula AxSiyNz, characterized in that Sr is used as cation, the silicide nitride being doped with trivalent Ce which acts as activator.
There is as yet no yellow-emitting fluorescent material of high efficiency which can be effectively stimulated in the region around 400 nm. The well known, normally used fluorescent material YAG:Ce can admittedly be effectively stimulated below 370 m and above 430 nm, but not in the region around 400 nm. Other Ce-doped garnets also exhibit only a slight ability to be stimulated in the range of use in question. It was therefore necessary to develop a completely different system.
According to the invention, the composition of the fluorescent material is selected such that it constitutes an Sr silicide nitride that is activated with trivalent Ce. The previously unknown fluorescent material Sr2Si5N8:Ce3+ absorbs efficiently in the near UV, in particular in the region of 370 to 430 nm, and has an efficient yellow luminescence. It is preferably activated by 1 to 10 mol % Ce (for Sr). In this case, the Sr can be replaced partially (advantageously up to at most 30 mol %) by Ba and/or Ca. A further embodiment constitutes a silicide nitride of the type SrSi7N10:Ce3+. In this case, too, the Sr can be replaced partially by Ba and/or Ca.
This fluorescent material is particularly well suited as a yellow component for stimulation by a primary UV radiation source such as, for example, a UV LED, or else a lamp. It is possible thereby to implement a light source emitting either in the white or yellow regions, as described similarly in WO 98/39807. A yellow-emitting light source is based on a LED primarily emitting UV radiation whose radiation is converted entirely into yellow light by a fluorescent material according to the invention.
In particular, this fluorescent material can be used in conjunction with a UV-LED (for example of the type InGaN), which generates white light by means of fluorescent materials emitting in the blue and yellow regions. Candidates for the blue component are known per se; for example, BaMgAl10O17Eu2+ (known as BAM) or Ba5SiO4(Cl,Br)6Eu2+ or CaLA2S4Ce3+ or else (Sr,Ba,Ca)5(PO4)3Cl:Eu2+ (known as SCAP). A red fluorescent material can be used, in addition, in order to improve the color of this system. (Y,La,Gd,Lu)2O2S:Eu3xe2x88x92, SrS:Eu2+ or else Sr2Si5N8:Eu2+ (not yet published, see EP-A 99 123 747.0) are particularly suitable.